Interpretive Summary: The iron in maize is not easily digested, so some populations that consume maize as a staple have widespread iron deficiency. Plants contain genes for hemoglobin, a form of iron that is easily digested, but the hemoglobin is present at very low levels. The goal of this experiment was to evaluate the potential of maize hemoglobin for improving maize as a source of dietary iron. First, maize hemoglobin was added to maize flour and determined to be as digestible as other well-digested sources of iron. Then, transgenic maize plants that produce extra maize hemoglobin in the seed were developed and evaluated. The resulting transgenic seeds were found to contain higher amounts of digestible iron than non-transgenic seeds. These results suggest that maize hemoglobin could be used to increase levels of digestible iron in maize. This observation will be useful to researchers trying to improve the nutritional quality of maize grain.

Technical Abstract:
Maize is an important staple crop in many parts of the world but has low iron bioavailability, in part due to its high phytate content. Hemoglobin is a form of iron that is highly bioavailable and its bioavailability is not inhibited by phytate. We hypothesize that maize hemoglobin is a highly bioavailable iron source and that biofortification of maize with iron can be accomplished by overexpression of maize globin in the endosperm. Maize was transformed with a gene construct encoding a translational fusion of maize globin and green fluorescent protein under transcriptional control of the maize 27kDa gamma zein promoter. Iron bioavailability of maize hemoglobin produced in E. coli and of stably transformed seeds expressing the maize globin-GFP fusion was determined using a Caco-2 cell culture model. Maize flour fortified with maize hemoglobin was found to have iron bioavailability that is not significantly different from that of flour fortified with ferrous sulfate or bovine hemoglobin and that is significantly higher than unfortified flour. Transformed maize grain expressing maize globin was found to have significantly higher iron bioavailability than untransformed seeds. These results suggest that maize globin may be an effective iron forticant and that overexpressing maize globin in maize endosperm may be a successful strategy to increase bioavailable iron content in maize.